Signals such as clock signals or pulse width modulated (PWM) signals have a pulse duty factor, which is also called a “duty cycle.” The pulse duty factor defines the ratio between the pulse duration, i.e., the active signal portion, and the period length of the signal. The non-active signal portion can also be referred to as an “off-time.”
When a switch mode regulator is operated as a down converter, a pulse duty factor that ideally corresponds to the ratio between the output voltage and input voltage occurs. For example, when an output voltage of 6 V is generated from an input voltage of 14 V, the resulting pulse duty factor is approximately 43%. With this pulse duty factor, the output stage of the switch mode regulator would be switched on for the first 43% of a clock period and switched off for the subsequent 57% of the clock period. An energy reservoir (e.g., a boost capacitor) is recharged in the off-time, i.e., when the output stage is switched off, and its charge is required again in the next period in order to switch on and/or hold a gate voltage of the output stage.
If the switch mode regulator is then operated close to the dropout region, i.e., if the input voltage is only slightly higher than the output voltage, the result is a pulse duty factor that approaches 100%. This operating state can occur in an automobile, for example, during the starting operation. This can cause a sharp drop in battery voltage.
If the charge state of the boost capacitor is not sufficiently high, the output stage can no longer be switched on in the subsequent clock period. The result thereof is then that the output voltage decreases, and the pulse duty factor is set by the regulator to 100%. As long as the boost capacitor does not have the necessary charge, however, the output stage cannot be switched on even when a pulse duty factor of 100% is set. Usually the boost capacitor is then recharged via an emergency current path. This mechanism does not engage, however, until the output voltage has decreased considerably and the pulse duty factor has been equal to 100% for some time, so that no further charge has also been taken from the boost capacitor for switch-on attempts or for unsuccessful switch-on operations. The result thereof is then that charge can no longer be taken from the boost capacitor for switch-on operations. This effect is perceptible as an undesired ripple or undesired oscillation in the output voltage, which makes the control circuit unstable.